1 Answer
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I am not completely sure, but my best guess is that Share quote feature requires that a book loaded on your Kindle be identifiable on Goodreads. This is trivial for Kindle books with their ASIN ids, but not so easy with sideloaded content. If I were in this situation, I would try finding out if I could manually assign ASIN id to a sideloaded book using Calibre or some such other tool (assuming the book exists on Goodreads database in the first place).